The present invention relates generally to purification and more particularly to the photolytic removal of phosgene impurity from boron trichloride using ultraviolet radiation.
Boron trichloride (BCl.sub.3) is used in the electronics industry, as an additive for high energy fuels, and as a catalyst, among other uses. The commercially available compound contains in excess of 0.8% of phosgene (COCl.sub.2) impurity. Efficient and economical BCl.sub.3 purification methods have not been reported to date. Industrial purification procedures generally exploit small differences in macroscopic physical properties between the substance of interest and the contaminants present. For example, fractional distillation, which relies on the difference in vapor pressures of the compounds to be separated at a given temperature, would be an obvious choice were it not for the unfortunately close vapor pressure curves of BCl.sub.3 and COCl.sub.2. This means that the two components would distill at approximately the same rate.
The method of the instant invention utilizes differences in the microscopic molecular properties between COCl.sub.2 and BCl.sub.3 to achieve high separation selectivity. That is, the enormous difference in ultraviolet absorption cross sections for COCl.sub.2 and BCl.sub.3 allows the phosgene to selectively absorb radiation from a mercury arc lamp, causing its photodecomposition, while the boron trichloride remains essentially untouched because it does not significantly absorb such radiation.
Two U.S. patents describe photolytic purification processes which are relevant to the instant invention:
1. In the U.S. Pat. No. 4,063,896, dated Dec. 20, 1977 to Merritt and Robertson, the inventors teach photolytic destruction of phosgene by what is essentially a laser pyrolysis. Particular wavelengths of carbon dioxide laser emission in the 10 .mu.m region of the infrared are absorbed by BCl.sub.3 which then transfers some of its vibrational excitation to the phosgene impurity, which does not absorb such wavelengths, by means of molecular collisions. The enormous temperature increase in the laser radiation path causes the phosgene to preferentially decompose since it is more thermally unstable than the BCl.sub.3. Such temperature increases, it might be argued, could be more reasonably obtained by heating the gas container. However, in the presence of hot walls it is likely that the BCl.sub.3 destruction would increase dramatically. That is, the cold walls and heated gas appear to allow significant phosgene decomposition with minimal BCl.sub.3 loss. The advantage and distinguishing feature of the method of the instant invention is that no laser source is required. A simple arc lamp with some output below 275 nm is sufficient. This feature renders our method much less expensive with regard to capital expenditure and operation and maintenance costs. Further, in our method, only the impurity molecules absorb radiation and consequently decompose with little energy wasted in heating the entire sample under irradiation. Therefore, the cost per impurity molecule removed is reduced. This is not to say, however, that an ultraviolet laser could not be used in place of the uv lamp.
2. In U.S. Pat. No. 4,146,449, dated Mar. 27, 1979 to Clark and Anderson, phosphine, arsine and diborane are photochemically removed from silane. Silane containing these impurities is irradiated by means of an ArF laser. A laser is necessary for their purification procedure since it is the only powerful light source available in the wavelength region of interest. That is, it is necessary to perform such photochemical separations at wavelengths where the difference between the absorption constants of the silane and the impurities is significant, which turns out to be around 193 nm, a wavelength where convenient and intense non-laser sources are not available. In our method, the relevant absorptions of BCl.sub.3 and COCl.sub.2 are well-separated, and the COCl.sub.2 absorbs wavelengths emitted from commonly available mercury arc lamps, while the BCl.sub.3 does not.